The problem of excessive heat build-up in an attic, defined as an enclosed space separating the roof of an architectural structure from an intermediate occupiable space, has long been known. The roofing material absorbs solar energy and conducts the heat to the interior rafters and decking of the attic, which in turn heat the air in the attic. Although air from the attic is generally not circulated through the living quarters, the trapped heat can penetrate insulation in the attic floor and heat the ceilings below, consequently raising the air temperature in the occupiable space. Without adequate air circulation in the attic, attic temperature can rise to 160.degree. F. on a hot, sunny day, substantially increasing the load on the air conditioning system for the space within the dwelling occupiable by human beings.
Contemporary methods of addressing this problem include natural and powered ventilation, or a combination of both. Natural ventilation often uses a "stack" effect, relying upon air flow caused by the difference in elevation between the lowest and highest portions of the attic. In one exemplary structure, screened openings are made in the roof near the peak and at the base, usually in the ridge and soffit, respectively. Warmer, lighter air rises to the top of the attic and is forced out of the opening near the peak by cooler, heavier outside air entering through the lower openings in the soffit.
Powered systems commonly use an electrically powered fan to expel hot air trapped in the attic. The expelled air is then replaced by outside air through the openings in the soffit, such as those described above. Attic Ventilation Control System in U.S. Pat. No. 4,251,026 to Siegel et al., provides a thermostatic control which activates an exhaust fan when the attic temperature reaches a predetermined point. The Positive Ventilation Cooling Augmenter of U.S. Pat. No. 4,870,832 to Crawley provides a collector and duct mounted above the heat exchanger of a home air conditioner. The system uses the exhaust pressure from the fan blowing external air across the heat exchanger, in combination with adiabatic expansion, to pressure-ventilate the attic. In each of the systems the outside air may itself be only sightly cooler than the air already within the attic, thereby limiting the cooling effect available.
Like the attic, a crawl space or basement located beneath the upper portions of the architectural structure typically occupied by human beings also requires good ventilation in the humid summer months, but for a different reason. A generally cooler temperature exists in such spaces, resulting primarily from the insulating effect provided by the occupied portions of the structure above the basement or crawl space, and the heat sink provided by the earth. The relative humidity of air, defined as the ratio of the amount of water vapor actually in the air to the greatest amount of water vapor that can theoretically be held in the air at the same temperature, increases as it enters the basement or crawl space and begins to cool. At lower temperatures, the maximum amount of water vapor the air can hold decreases and any excess moisture condenses on cooler surfaces, resulting in mildew and in extreme cases rotting wood, rusted metal, and cracked plaster. The Household Dehumidifier of U.S. Pat. No. 5,092,520 to Lestage has a humidity controlled fan installed in a duct, for exhausting the coolest, dampest air from a basement or crawl space to the outside. No beneficial use, however, is made of the naturally cooled air. The Supplemental Heating and Cooling System of U.S. Pat. No. 4,182,401 to Pinnell et al. circulates cool air between a basement and a living space in the cooling season to reduce the load on an air conditioning system, but fails to beneficially use that same cool air to affect attic temperatures.
A number of patents show specially built solar heated structures which utilize the earth as a heat sink, or substitute a thermal storage chamber insulated from the earth and made from dense materials such as concrete or rock, in order to cool air within the structure. For example, the Solar Efficient Structure of U.S. Pat. No. 4,498,526 to Arenas provides a closed system for circulating air between an occupied space and an attic through a duct network positioned in thermal contact with the ground. Since the cooled air is exhausted to both the attic and the occupied space however, a dehumidifier is required to insure against condensation in the attic and excessive humidity in the occupied space.
Similarly, the Environmentally Heated and Cooled Building of U.S. Pat. No. 4,501,999 to Granger et al. shows a rock bed thermal storage chamber insulated from the earth, having a duct and fan network for circulating air through a previously cooled storage chamber in order to cool the attic and occupied space (mode 6). Prior to using the storage chamber to cool the air however, the rock bed must have been prepared by passing cool outside air through it (mode 8), since the chamber is insulated from the earth as a heat sink. An alternative operation for warm days (mode 5), draws outside air directly into the occupied space and attic without passing it through the storage chamber, again requiring that the outside air be relatively cool.
The Warm Climate Solar Building of U.S. Pat. No. 5,121,789 to Scharf also uses a thermal storage device comprising a duct network embedded in a concrete slab insulated from the earth. Cool night time air is drawn from outside the structure through the duct network and into an attic by a timer controlled fan. During hot summer days however, the fan is shut off and the outside air intake is closed, resulting in little or no cool air flowing into the attic. The occupied space is cooled through conduction of heat to the slab itself.
Each of the above described solar structures utilize cooling systems which would be difficult and cost prohibitive to install in an existing structure, and which fail to provide for dehumidification of a basement or crawl space.
In recent years it has been discovered that in some areas of the country naturally occurring radon seeps from the soil and tends to accumulate in basement areas. Radon gas is radioactive and a known carcinogen when inhaled by humans. A dangerous condition can occur if the radon gas is permitted to become concentrated within an occupied area. If air containing high levels of radon gas is drawn into the attic, it could leak into the occupied areas of the building.